Add _pydecimal and _decimal stubs (#4003)

tests/stubtest_whitelists/py35.txt

@@ -21,7 +21,6 @@ codecs.StreamRecoder.seek
 collections.Reversible
 collections.UserString.maketrans
 ctypes.CDLL.__init__
-decimal.Decimal.as_integer_ratio
 fractions.Fraction.__new__  # private _normalize param was made keyword-only in Python 3.6
 gettext.NullTranslations.npgettext
 gettext.NullTranslations.pgettext

tests/stubtest_whitelists/py38.txt

@@ -66,8 +66,6 @@ copy.PyStringMap
 dataclasses.dataclass
 dataclasses.field
 dataclasses.replace
-decimal.Decimal.from_float
-decimal.setcontext
 email.message.MIMEPart.as_string
 enum.Enum._generate_next_value_
 functools.partialmethod.__get__

tests/stubtest_whitelists/py3_common.txt

@@ -154,7 +154,6 @@ curses.COLS
 curses.LINES
 dbm.error
 dbm.ndbm
-decimal.DecimalException.handle
 difflib.SequenceMatcher.__init__
 distutils.archive_util.make_archive
 distutils.archive_util.make_tarball
@@ -523,6 +522,7 @@ zlib.compressobj
 # ==========
 # Whitelist entries that cannot or should not be fixed
 # ==========
+_pydecimal.*  # See comments in file
 ast.NodeVisitor.visit_\w+  # Methods are discovered dynamically, see #3796
 # Weird special builtins that are typed as functions, but aren't functions
 builtins.copyright